Move _unittest.yml to linux_job_v3

[huydhn]

First of six splitting up #22107, which was too large to review in one piece. Each PR stacks on the previous one.

_docker-image.yml, which this depends on, now lands in #22106 instead.

_unittest.yml is its first caller, so pull.yml and trunk.yml exercise the resolution on every PR. The docker-image input still takes ci-image:<name>, so its own callers are unchanged; this workflow expands that into the full ECR reference.

Authored with Claude Code.

[pytorch-bot]

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✅ You can merge normally! (1 Unrelated Failure)

As of commit c417bb7 with merge base b46bd04 ():

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (trunk failure)

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[huydhn]

The two unittest / linux failures are not in this diff — they are linux_job_v3's own first step:

rm: cannot remove '/__w/executorch/executorch': Permission denied

The GH runner image creates GITHUB_WORKSPACE as uid 1001, our ci-image ends with USER ci-user at uid 1000 ("Same as ec2-user"), so that user cannot unlink it and every later step skips.

linux_job_v2 ran the same cleanup under sudo by default, gated on its no-sudo input; v3 dropped both, so this is a regression that only surfaces with a non-root image — which is why cuda.yml's v3 jobs on pytorch/almalinux-builder never hit it, and why this PR is the first to. pytorch's own images have the identical USER jenkins/uid 1000 setup and work around it with sudo chmod -R 777 "$GITHUB_WORKSPACE" as the first step of setup-linux, which they can do because they own the workflow; v3 hardcodes Clean workspace first.

Fix: pytorch/test-infra#8665. The last commit here temporarily points _unittest.yml at that branch to prove it, and comes off before landing.

[huydhn]

The three test_gather_benchmark_configs.py failures are real, and this PR surfaces a latent test bug rather than causing one.

gather_benchmark_configs.py's set_output() writes the matrix to $GITHUB_OUTPUT when that is set, and only prints it when it is not — falling back to printing solely on PermissionError/FileNotFoundError, with a comment saying "Fall back to printing in case of permission error in unit tests". The three CLI tests assert against result.stdout, so they depend on that fallback firing.

Under v2 the script ran through a docker exec where $GITHUB_OUTPUT pointed at a host path the container could not see, so the fallback fired and the assertions passed. v3 runs the script in the same container as the runner, so the file is present and writable — the matrix goes to the file and stdout has nothing to match.

Verified directly:

GITHUB_OUTPUT set (v3)                 rc=0  stdout has matrix: False
GITHUB_OUTPUT removed (fixed test)     rc=0  stdout has matrix: True

Last commit drops the variable from the environment the tests hand to the CLI, so they assert what they mean wherever they run. I fixed the test rather than the script, since the script's behaviour is correct — writing to $GITHUB_OUTPUT in CI is the point.